1. Field of the Invention
This invention pertains to improved spiral-wound pressure membrane modules comprising a central permeate conduit around which is wrapped in swiss-roll fashion one or more membrane envelopes as disclosed for example in U.S. Pat. Nos. 3,367,504 and 3,417,870. Each envelope comprises a pair of generally asymmetric or thin film membranes separated from each other by a permeate spacer adapted to afford mechanical support to the membranes and to carry fluid permeating through the membranes along the spiral to the central permeate conduit. The membranes in the pair are typically sealed to each other along three edges with an adhesive or by heat or solvent welding. The remaining edge of the pair may remain open to permit permeate to flow into the central permeate conduit. The latter is generally circular in cross-section, the better to withstand the pressure applied to the module. The wraps in the spiral are typically separated from each other by so-called feed side spacers adapted to support the membranes and facilitate mass-transport to the membrane surfaces. Such spiral wound modules were apparently first used for the production of potable water from brackish water but now are also commonly used for ultrafiltration, microfiltration, gas-separations and other processes. Generally these pressure-driven membrane separation processes do not provide perfect separation of the components in the fluid processed and the permeate may need to be reprocessed by one or more additional stages of the same (or other) pressure-driven membrane processor by one or more auxiliary processes. For example in the case of demineralization of saline water, the permeate may contain from about one to about twenty percent or more of the mineral concentration present in the feed saline water and in addition low molecular weight organics (such as trihalomethanes), microorganisms, viruses, endotoxins and pyrogens. The permeate may also contain concentrations of heavy metal ions such as lead, cadmium, mercury, chromate, radiostrontium and the like which render the permeate as is unsuitable for the intended use, e.g. for safe drinking water, pharmaceutics, rinsing of large scale integrated circuits, etc. It may be necessary to pass the permeate through additional process apparatus, for example, activated carbon and/or a mixed bed ion-exchange resin bed to remove residual organics. Such auxiliary apparatus obviously adds cost and complexity to the otherwise elegant piezo-membrane process. Activated carbon and ion-exchange resins are examples respectively of adsorption and adsorption materials typically used in fluid processing. Herein these and like materials are referreed to generally as sorbing means or sorbents. Such sorbents are most efficiently used in countercurrent processes, the fluid to be processed contacting first that portion of the sorbing means having the highest loading of sorbate and finally that portion having the lowest loading. Such contacting is best carried out in columns or beds in which the fluid processed enters at one end and exits at the other.
According to the instant invention, such column replaces the conventional permeate conduit of pressure driven spiral wound modules known in the prior art, collection means in the integrated module conveying the membrane permeate to substantially the same cross-section of the sorbent column thereby providing the efficiency of counter-current contacting with the simplicity of a single integrated module. 2. Description of the Prior Art
Spiral wound pressure membrane modules for reverse osmosis desalination of brackish water are disclosed in U.S. Pat. Nos. 3,367,504 and 3,417,870 among others. Similar construction are used for example in ultrafiltration, microfiltration and membrane gas-separations. The membrane envelopes containing permeate separators and inter-leaved with feed spacers are wrapped around a central core typically perforated along lines which communicate with the interior of each envelope. The permeate therefore flows more or less uniformly into the core along the entire length in contact with the envelope. Hence the core is not really suitable as a container for an auxiliary sorbent and as a consequence trains of separation processes are coupled together in separate containers. For example in the case of home water systems designed to make safe drinking water, typically a low pressure reverse osmosis module designed to remove 80 to 90 percent of dissolved minerals and organic compounds is followed by an activated carbon filter to remove additional organics. In the case of point-of-use rinse water systems for electronic components an ultrafiltration module may be followed by a mixed bed ion-exchanger.